Hydroxyl radical production in the ozonolysis of catechol at the air-solid interface

Dihydroxybenzenes such as catechol are major aromatic constituents of biomass-burning emissions and are readily deposited onto atmospheric particle surfaces, where they undergo heterogeneous oxidation. Here, hydroxyl radical (HO˙) production during the ozonolysis of catechol at the air-solid interface is quantified using an indirect fluorescent probe approach. Thin films containing catechol and molecular scavengers were exposed to ozone (0.5-112 ppmv) under controlled relative humidity (RH = 0-85%) conditions, and HO˙ formation was inferred from the production of hydroxylated probe products. Significant HO˙ production was observed under all conditions, with interfacial production rates increasing systematically with both ozone concentration and RH, reaching 3.1 × 10¹⁰-1.2 × 10¹² molecules cm^-2 s^-1 under humid conditions (85% RH). Quantitative kinetic analysis reveals a linear dependence of HO˙ production on [O₃(g)], consistent with pseudo-first-order behavior under oxidant-limited conditions, and enabling derivation of an effective HO˙-forming uptake probability. Concurrently, the interfacial reaction velocity increases exponentially with RH, reflecting enhanced oxidant uptake, interfacial transport, and reactive turnover in hydrated films. By relating the measured reaction velocity to kinetic gas theory, an effective HO˙-forming uptake probability of γ_HO˙,eff ≈ (5.4 ± 1.9) × 10^-10 is obtained. Comparison with independently reported ozone uptake coefficients for catechol films (γ_O3) yields an inferred HO˙ formation efficiency of Y_HO˙ = (3.4 ± 1.2) × 10^-5, indicating that hydroxyl radical production proceeds through a measurable secondary pathway of heterogeneous ozonolysis. These results demonstrate that surface-mediated oxidation of aromatic films constitutes a non-photochemical source of hydroxyl radicals under atmospherically relevant conditions. The strong dependence on relative humidity suggests that interfacial hydration modulates both ozone uptake and radical formation, providing a quantitative framework for evaluating this process as a sensitivity pathway in multiphase atmospheric oxidation chemistry.